I sugguested smaller mysis for juveniles because they are more likely to go for small size food. As they grow bigger, specially the female, they will need more meaty food, frozen food is also more nutritious than flakes, and contains more HUFA (highly unsaturated fatty acid).
Both Formula One and Prime Reef has the same ingredients, except Formula One contains gel binder, and the other doesn't. I personally prefer Prime Reef, as it doesn't contain the binder, you get uneven size chunks of food, and it's great for feeding your entire tank from large fish to inverts, even filter-feeding inverts.